Question: Simplify the following expression: $a = \dfrac{3}{10z} - \dfrac{5}{7z}$
Answer: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $10z$ and $7z$ $\lcm(10z, 7z) = 70z$ $ a = \dfrac{7}{7} \cdot \dfrac{3}{10z} - \dfrac{10}{10} \cdot \dfrac{5}{7z} $ $a = \dfrac{21}{70z} - \dfrac{50}{70z}$ $a = \dfrac{21 -50}{70z}$ $a = \dfrac{-29}{70z}$